Two-dimensional, phase modulated lattice sums with application to the Helmholtz Green's function

A class of two-dimensional phase modulated lattice sums in which the denominator is an indefinite quadratic polynomial Q is expressed in terms of a single, exponentially convergent series of elementary functions. This expression provides an extremely efficient method for the computation of the quasi-periodic Green's function for the Helmholtz equation that arises in a number of physical contexts when studying wave propagation through a doubly periodic medium. For a class of sums in which Q is positive definite, our new result can be used to generate representations in terms of Θ-functions which are significant generalisations of known results

Water waves over arrays of horizontal cylinders: band gaps and Bragg resonance

The existence of a band-gap structure associated with water waves propagating over infinite periodic arrays of submerged horizontal circular cylinders in deep water is established. Waves propagating at right angles to the cylinder axes and at an oblique angle are both considered. In each case an exact linear analysis is presented with numerical results obtained by solving truncated systems of equations. Calculations for large finite arrays are also presented, which show the effect of an incident wave having a frequency within a band gap – with the amount of energy transmitted across the array tending to zero as the size of the array is increased. The location of the band gaps is not as predicted by Bragg’s law, but we show that an approximate determination of their position can be made very simply if the phase of the transmission coefficient for a single cylinder is known

The finite dock problem

The scattering of water waves by a dock of finite width and infinite length in water of finite depth is solved using the modified residue calculus technique. The problem is formulated for obliquely incident waves and the case of normal incidence is recovered by taking an appropriate limit. Exciting forces and pitching moments are calculated as well as reflection and transmission coefficients. The method presented in this paper takes account of the known solution for the scattering by a semi-infinite dock to produce new and extremely accurate approximations for the reflection and transmission coefficients as well as a highly efficient numerical procedure for the solution to the full linear problem

The interaction of waves with horizontal cylinders in two-layer fluids

We consider two-dimensional problems based on linear water wave theory concerning the interaction of waves with horizontal cylinders in a fluid consisting of a layer of finite depth bounded above by a free surface and below by an infinite layer of fluid of greater density. For such a situation time-harmonic waves can propagate with two different wavenumbers K and k. In a single-layer fluid there are a number of reciprocity relations that exist connecting the various hydrodynamic quantities that arise. These relations are systematically extended to the two-fluid case. It is shown that for symmetric bodies the solutions to scattering problems where the incident wave has wavenumber K and those where it has wavenumber k are related so that the solution to both can be found by just solving one of them. The particular problems of wave scattering by a horizontal circular cylinder in either the upper or lower layer are then solved using multipole expansions

Bound states in coupled guides. II. Three dimensions.

We compute bound-state energies in two three-dimensional coupled waveguides, each obtained from the two-dimensional configuration considered in part I by ro- tating the geometry about a different axis. The first geometry consists of two concentric circular cylindrical waveguides coupled by a finite length gap along the axis of the inner cylinder and the second is a pair of planar layers coupled laterally by a circular hole. We have also extended the theory for this latter case to include the possibility of multiple circular windows. Both problems are formulated using a mode-matching technique, and in the cylindrical guide case the same residue calcu- lus theory as used in I is employed to find the bound-state energies. For the coupled planar layers we proceed differently, computing the zeros of a matrix derived from the matching analysis directly

Bound states in coupled guides. I. Two dimensions.

Bound states that can occur in coupled quantum wires are investigated. We consider a two-dimensional configuration in which two parallel waveguides (of dif- ferent widths) are coupled laterally through a finite length window and construct modes which exist local to the window connecting the two guides. We study both modes above and below the first cut-off for energy propagation down the coupled guide. The main tool used in the analysis is the so-called residue calculus technique in which complex variable theory is used to solve a system of equations which is derived from a mode-matching approach. For bound states below the first cut-off a single existence condition is derived, but for modes above this cut-off (but below the second cut-off), two conditions must be satisfied simultaneously. A number of results have been presented which show how the bound-state energies vary with the other parameters in the problem

On step approximations for water-wave problems

The scattering of water waves by a varying bottom topography is considered using two-dimensional linear water-wave theory. A new approach is adopted in which the problem is first transformed into a uniform strip resulting in a variable free-surface boundary condition. This is then approximated by a finite number of sections on which the free-surface boundary condition is assumed to be constant. A transition matrix theory is developed which is used to relate the wave amplitudes at fm. The method is checked against examples for which the solution is known, or which can be computed by alternative means. Results show that the method provides a simple accurate technique for scattering problems of this type

The existence of Rayleigh-Bloch surface waves

Rayleigh-Bloch surface waves arise in many physical contexts including water waves and acoustics. They represent disturbances travelling along an infinite periodic structure. In the absence of any existence results, a number of authors have previously computed such modes for certain specific geometries. Here we prove that such waves can exist in the absence of any incident wave forcing for a wide class of structures

On the excitation of a closely spaced array by a line source

An infinite row of periodically spaced, identical rigid circular cylinders is excited by an acoustic line source, which is parallel to the generators of the cylinders. A method for calculating the scattered field accurately and efficiently is presented. When the cylinders are sufficiently close together, Rayleigh–Bloch surface waves, which propagate energy to infinity along the array are excited. An expression is derived which enables the amplitudes of these surface waves to be computed without requiring the solution to the full scattering problem

An interaction theory for scattering by defects in arrays

Wave scattering by an array of bodies that is periodic except for a finite number of missing or irregular elements is considered. The field is decomposed into contributions from a set of canonical problems, which are solved using a modified array scanning method. The resulting interaction theory for defects is very efficient and can be used to construct the field in a large number of different situations. Numerical results are presented for several cases, and particular attention is paid to the amplitude with which surface waves are excited along the array. We also show how other approaches can be incorporated into the theory so as to increase the range of problems that can be solved